1. Field of the Invention
The present invention generally relates to a surface acoustic wave device using a piezoelectric substrate and a low expansion material. The present invention also relates to a filter device using such a surface acoustic wave device and a method of producing the same.
2. Description of the Related Art
Nowadays, the surface acoustic wave device is widely used as a band-pass filter of a portable phone. The surface acoustic wave filter enables a compact, less-expensive filter and resonator, and is a key component for downsizing the communication devices such as portable phones.
The filter using the surface acoustic wave device is required to have higher performance as the portable phone technically and functionally advances. Generally, the surface acoustic wave filter has a frequency characteristic that depends on temperature. It is required to improve the temperature stability of the surface acoustic wave filter.
As is known lithium tantalate (LiTaO3: LT), which is in heavy usage as a material for the substrate of the surface acoustic wave device, is a piezoelectric material with a large electromechanical coupling factor.
Piezoelectric material having a large electromechanical coupling factor generally exhibits poor temperature stability. In contrast, piezoelectric material having good temperature stability such as quartz has poor electromechanical coupling coefficient. Therefore, the surface acoustic wave device with the LT substrate is advantageous to realizing a broadband filter characteristic, although it does not have comparatively good temperature stability.
It is strongly desired to realize a material that has a large electromechanical coupling coefficient and good temperature stability. There are some proposals to attempt to realize such material. Some examples of such proposals are illustrated in FIGS. 1A through 1D.
FIG. 1A shows a conventional surface acoustic wave device 100. This device is described in, for example, Yamanouchi et al., IEEE Trans. On Sonics and Ultrasonics., vol. SU-31, pp. 51–57, 1984. Hereinafter, the device 100 is referred to as the first prior art. The surface acoustic wave device 100 according to the first prior art has a piezoelectric substrate 11 made of lithium niobate (LiNbO3: LN) or LT on which comb-like electrodes are formed. The surface of the piezoelectric substrate 100 is coated with a quartz film 14, which as a temperature coefficient opposite to that of LT or LN. The quartz film 14 functions to cancel the temperature characteristic of the piezoelectric substrate 11, so that the temperature stability can be improved.
FIG. 1B shows another conventional surface acoustic wave device 200, which is described in Japanese Patent No. 25168171. Hereinafter, the device 200 is referred to as the second prior art. A polarization-inverted layer 15 is provided on the surface of the LT or LN substrate 11 on which the comb-like electrodes 12 are formed. The polarization-inverted layer 15 has a thickness less than the wavelength of a surface acoustic wave (SAW) that travels on the surface of the substrate 11. The electric field short-circuiting effect of the polarization-inverted layer 15 is used to improve the temperature stability.
FIG. 1C shows yet another conventional surface acoustic wave device 300 described in, for example, Japanese Laid-Open Patent Application No. 11-55070 or Ohnishi et al., Proc. Of IEEE Ultrasonics Symposium, pp. 335–338, 1998. The device 300 is referred to as the third prior art. The device 300 has a thinner piezoelectric substrate 11a than the substrate 11 shown in FIGS. 1A and 1B, and another substrate 16, which is thicker than the substrate 11a and is made of a low expansion. The substrate 16 is directly bonded to the piezoelectric substrate 11a. The low-expansion substrate 16 suppresses expansion and compression of the piezoelectric substrate 11a caused by temperature change, so that the temperature stability can be improved.
However, the surface acoustic wave device 100 according to the first prior art has a disadvantage in that there is a difficulty in forming the quartz film 14 having an even thickness. The quarts film 14 is provided on even the comb-like electrodes 12, this increasing the propagation loss of the surface acoustic wave.
The surface acoustic wave device 200 according to the second prior art has a difficulty in controlling the depth of the polarization-inverted layer. This brings about difficulty in fabrication and degrades the yield. The surface acoustic wave 300 according to the third prior art requires a mirror surface for satisfactory bonding. However, the mirror surface subject to bonding may cause reflection of a bulk wave, which may degrades the filter characteristic.
FIG. 1D shows a further conventional surface acoustic device 400 described in, for example, Japanese Laid-Open Patent Application No. 2001-53579. The device 400 is hereinafter referred to as the fourth prior art. The back surface of the piezoelectric substrate 11a opposite to the front surface thereof on which the comb-like electrodes 12 are formed is made rough. A reference numeral 18 indicates such a rough surface of the piezoelectric substrate 11a. The piezoelectric substrate 11a thus formed is bonded to the low-expansion substrate 17 b means of an adhesive layer 17.
The fourth prior art may improve the filter characteristic. However, the adhesive layer 17 interposed between the piezoelectric substrate 11a and the low-expansion substrate 16 may prevent the aforementioned improvement in the temperature characteristic. Further, the adhesive force at the interface is weakened, and the improvements in the temperature stability are degraded.